Where do we go from here?

The Moken children on the scattered islands of the Andaman Sea have a remarkable skill. Years of plunging beneath the waves to find food, in the form of fish and mussels, has given them an extraordinary ability to see underwater. It is a skill that is crucial for their survival, and one that sent ripples through the scientific world when it was reported four years ago.

The question some scientists asked was whether the Mokens' ability to see while they foraged on the sea bed was down to evolution. If true, it would be a remarkable example of genetic adaptation in modern humans. Studies later that year, however, showed that given enough practice, other children could repeat the skill by tightly constricting their pupils. The Mokens' trick, it seems, is more about learning than genetics.

The question of whether modern humans are evolving has not gone away, though. This week, scientists added to a growing pile of papers that indicate human evolution is not only continuing, but may be accelerating at an unprecedented pace. Where this will take us has become one of the most contentious questions in evolutionary biology.

"We've been almost indoctrinated with this notion that human evolution stopped long ago," says Henry Harpending, an anthropologist at the University of Utah. "Part of the problem was that in the 1950s, social science, in order to validate itself, needed a homogenous humanity, and if you read the literature, it's as if there was a truce. It seems biologists wouldn't even talk about it."

In an essay entitled The Spice of Life published in 2000, the late evolutionary biologist Stephen Jay Gould reinforced the idea that thanks to improvements in medicine, shelter and, for many, plentiful food, humans had all but stepped off the evolutionary ladder. "Natural selection has almost become irrelevant," he wrote. "There's been no biological change in humans in 40,000 years or 50,000 years. Everything we call culture and civilisation we've built with the same body and brain."

There are other reasons that scientists were willing, at least for a time, to leave the question of recent human evolution unasked. Thanks to Darwin's half-cousin, Sir Francis Galton, who advanced theories of eugenics and racial superiority, investigations into recent human evolution and the inevitable differences between disparate populations were for a long time only for the brave or foolish. With the scientific tools of the 21st century, this has changed.

This week, Harpending's group published details of a study that asked how much humans have evolved in the past 80,000 years, a period that includes the exodus of humanity from Africa. The answer, they concluded, was an awful lot. They identified a rapid increase in evolution, as our ancestors adapted first to harsh latitudes with miserable climates, then to farming, which revolutionised the human diet. Harpending's group studied the DNA of four distinct groups around the planet: Japanese, Han Chinese, Europeans and Yoruba in Africa. They found that nearly 2,000 genes, or 7% of the genome, have been subjected to recent natural selection.

Evolution in its most basic sense is merely a shift in how common certain variants of genes are in a population. But evolution can occur through different processes. Natural selection drives evolution if a gene improves an organism's ability to pass on its DNA - usually by having children. But genes can also become more common if they cause traits or behaviours that are more attractive to the opposite sex.

Harpending's study highlighted some already well-known genetic changes. One was the spread of LCT, a gene that allows us to digest lactose, a sugar in milk, beyond our childhood years. In China and Africa, few people have the gene, but in Sweden and Denmark it has become almost ubiquitous in the past 10,000 years. Harpending believes the gene spread through Europe after the advent of dairy farming, and may have spurred some of history's population expansions, including the settling of Indo-Europeans from north-west India through Persia. Those who could digest milk later in life had more energy and could conquer a larger area, he argues.

Other genes came to the fore due to the spread of life-threatening diseases. One gene, called G6PD, is emerging in African populations. Although it harms people by causing anaemia, it harms malaria more, and so confers a net benefit. Likewise a gene called CCR5-32, which improves resistance to HIV infection, is becoming more prevalent.

Harpending's study also threw up some real puzzles. In Asia, a gene for coarse hair is emerging. So is one for dry earwax. Harpending is guessing, but says the latter may have emerged because it performs another job, such as reducing body odour. In short, while there is proof of recent evolution, precisely where it is going is less clear.

Harpending's technique can pick up evolutionary changes as recent as 2,000 years ago. What is controversial is his interpretation of the data. While he believes humanity continues to evolve at the speed of our distant ancestors, his critics say this is impossible to prove.

Harpending isn't the only one to have found evidence of recent evolutionary change. Bruce Lahn, a geneticist at the University of Chicago in Illinois, has reported evolutionary changes in the brain, caused by two new genes involved in brain growth that emerged around 37,000 and 5,800 years ago. In 2001, Ian Owens, a geneticist at Imperial College, studied thousands of twins, concluding that natural selection was favouring more teenage pregnancies. His study revealed that the "reproductive fitness", or ability to pass on genes, of Catholic women was 20% higher than average, leading to a murky entwining of culture and evolution where the latter may even reinforce the first. "We know religiosity is heritable," says Harpending. "It's a small effect, but it's there."

But while other scientists agree that evolution continues, they are divided on the role played by natural selection - or whether it has a role at all.

In his book The Language of the Genes, the University College, London, geneticist Steve Jones argues that human evolution has slowed dramatically since we settled and developed farming. The reason, he says, is that we have insulated ourselves from the upheavals we faced in the distant past which only the hardiest survived.

"If you look at the way of life of modern people, in many ways we've gone back to the savannah, in the sense that we all now take the tropics with us. We sit in warm offices with bright lights and wear warm clothes. We no longer have to slave over our little patch of land in an awful climate as an early farmer would have. When we go hunter-gathering, we go down to Sainsbury's," he says.

The development of sanitation, antibiotics and other medicines have protected many of us from the epidemics that culled millions and left only a fitter, genetically different population behind.

Another huge change is an averaging-out of the number of children people have. Natural selection can only occur when people have different numbers of children, because if everyone has the same number, all genes are equal.

But despite natural selection losing much of its power over us, Jones concedes we are still evolving by other mechanisms. Modern life can in fact drive degenerative or reverse evolution, where genes that make us resistant to starvation, or to once life-threatening disease and infections, and even ones that give us good sight and hearing, lose their value and disappear. "Our improvements in public health and survival are going to face us with fresh challenges. In the past, if you had bad eyesight you might have starved or been eaten by a sabre-toothed tiger," he says.

As for the future, few scientists are willing to speculate on how humans will change, but there is agreement that mass transport will drive unprecedented mixing between previously isolated populations. Genes that have been separated for tens of thousands of years will be reunited in combinations that have never been seen before. One consequence will be an evening-out of skin colour, hair colour and other traits, which is expected to happen quickly over the next few centuries.

"People often think evolution means greater divergence, but now it's going to become a grand homogenisation, a triumph of the average," says Jones.

Harpending's paper found that as races settled in different regions, they began to diverge as they evolved to suit their environment. It is this isolation that, taken to its extreme, leads to the creation of new species. While there's no chance of that happening to humans on Earth, biologists say it would be a possibility if we live on other planets.

"I'm confident that unless we go off and colonise some other planet, we could come back in 5,000 years and people would look much the same, but their intellectual abilities might be quite different," says Chris Wills, a geneticist at the University of California, San Diego. He believes our brains will be the main focus of future evolutionary changes. "The essence of human beings is their intelligence and at the present time people have an enormous range of different abilities. My prediction is one of the ways we'll evolve is to add genes that increase our range of abilities," he says.

Many evolutionary biologists now admit they are dazzled by the latest data coming out. Mostly, it says we are changing and those changes involve more of our genetic code than ever thought. "Now we have to ask why that is," says Wills. "The question is, what is going to happen to us in the future?"

In the genes

Fertility

Some scientists believe women will divide into two groups: those primed to be teenage mothers, and those who conceive later in life. Contraception could select out reluctant parents, while caesareans could increase the number of larger babies.

Intelligence

Natural selection has increased the intelligence of Ashkenazi Jews over the past 1,000 years, claims Henry Harpending of the University of Utah. And two forms of genes involved in brain growth have spread recently.

Immunity

Resistance genes for many diseases, such as malaria and, to a lesser extent, the HIV virus, have emerged, but genes that protect against bacteria may vanish as antibiotics fend off infections.

Appearance

Skin and hair colour, facial and physical features such as body shape are likely to even out across cultures.

Nutrition

Humans are still adapting to modern diets, and some scientists believe genes are now spreading to help us thrive on high-carbohydrate diets. Adapting could see falls in obesity.